Many processes for preparing polysulfides adapted to be used as extreme-pressure additives for lubricants are disclosed in the prior art.
In particular, U.S. Pat. Nos. 3,471,404 and 3,697,499 disclose a process comprising the following main steps of :
(1) reacting sulfur monochloride with an excess of a C.sub.2 to C.sub.5 olefin,, particularly isobutene, at a temperature from 20 to 80.degree. C., so as to form an adduct,
(2) reacting the adduct of the first step with an alkali metal sulfide (preferably sodium sulfide) and elemental sulfur, in a ratio from 1.8 to 2.2 moles of metal sulfide per gram-atom of sulfur, the proportion of alkali metal sulfide being from 0.8 to 1.2 mole per mole of adduct , and the reaction being performed in the presence of an alcohol or of a hydro-alcoholic solvent, at reflux, and
(3) reacting the resultant product, which contains 1-3% of chlorine, with an inorganic base in aqueous solution, at reflux until the residual chlorine content of the product is reduced to less than 0.5%.
These prior art patents indicate that the sulfur content of the obtained products may be from 40 to 60% by weight. In fact, it is usually close to 46% by weight. These products may be used as extreme-pressure additives for lubricating oils, transmission fluids or greases, the lubricating bases consisting of mineral oils and of certain synthetic oils.
U.S. Pat. No. 4,204,969 discloses a rather similar process for preparing olefin polysulfides for use as extreme-pressure additives for lubricating oils. This process comprises the main steps of:
(1) reacting, at about 30-100.degree. C., sulfur monochloride with a C.sub.3 -C.sub.6 aliphatic monoolefin (generally isobutene) preferably in the presence of a promoter consisting of a lower alcohol, so as to form an adduct,
(2) reacting said adduct with sulfur and sodium sulfide (prepared for example from NaOH, NaHS and/or H.sub.2 S) in a ratio of 0.1-0.4 gram-atom of sulfur per mole of sodium sulfide, in a hydroalcoholic medium, at a temperature from 50.degree. C. to reflux temperature, and recovering the resultant product, without treatment by means of a base. The lower alcohol, of 1-4 carbon atoms, is generally present in an amount by weight of 0.1-0.5 parts per part of water.
In the sole example of said patent, it is mentioned that the product has a 49% by weight sulfur content and a viscosity at 100.degree. F. (37.8.degree. C.) of 8.6 mm.sup.2 /s (cSt), but the analysis indicates that said product has a residual chlorine content of about 0.25% by weight.
It is apparent that the isobutylene polysulfides prepared according to the prior art in reaction media containing substantial amounts of water, always contain residual chlorine amounts higher than 0.1% by weight, the complete removal of chlorine being difficult as a result of the insolubility of the adduct in the reaction medium, which results in an incomplete heterogeneous phase reaction.
In most of the described processes, water is generally introduced into the reaction medium in the presence of a minor amount of alcohol (particularly isopropanol), for dissolving the alkali sulfide, itself mostly containing a substantial amount of (at least 40% by weight for industrial Na.sub.2 S).
It is however possible to increase the dissolving power of the reaction medium with respect to the adduct by substituting the dissolution water of the hydrated alkali sulfide with a lower alcohol, but the required alcohol amount is then incompatible with satisfactory economical industrial production.
It is further known that, for toxicology and ecology purposes, a new legislation, in course of elaboration, concerning additives for lubricants, will probably prescribe, in the next few years, a maximum chlorine content substantially lower than 0.1% by weight for these products.